1. Field of the Invention
This invention relates to stabilized thermoplastic molding compositions based on polyvinylchloride or polymers essentially containing vinylchloride.
2. Description of Related Art
In the molding of thermoplastic polymers to tubes, bottles, profiles, films, etc. by processes including extrusion, injection molding, blow-molding, deep-drawing and calendering, degradation of the plastic material involved can occur due to the high temperatures encountered. Degradation is manifested by undesirable discoloration of the plastic and in a deterioration in its mechanical properties.
To prevent, or at least minimize this, stabilizers are added to the polymers before molding to counteract degradation. The heat stabilizers which are used for polyvinylchloride and for copolymers essentially containing vinylchloride are generally inorganic and organic lead salts, organic antimony compounds, organotin compounds and also cadmium/barium carboxylates and phenolates. The foregoing metal compounds are normally termed primary stabilizers, with secondary stabilizers or co-stabilizers often being added to the primary stabilizers to enhance their effectiveness. Additional information relating to the heat stabilizers normally used for vinylchloride polymers can be found in technical literature, as, for example Encyclopedia of Polymer Science and Technology, Vol. 12, Wiley-Interscience Pub., New York, 1970, pages 737 to 768.
The stabilizers mentioned heretofore have, in general, been acceptable in actual use. However, certain polyvinylchloride mixtures yield products having undesirable properties, such as unsatisfactory initial and/or long-term stability values. These values cannot be corrected or improved by modifying the presently known stabilizers or stabilizer mixtures or by increasing the quantities of stabilizer used. In addition, certain objections of a toxicological nature have been raised regarding the use of lead, antimony and cadmium compounds in certain fields. On the other hand, although many organotin compounds are toxicologically safe, their high price is an obstacle to their widespread use. For this reason, attempts have long been made to replace these compounds by safer, inexpensive compounds.
For example, certain fatty acid salts, aromatic carboxylates and phenolates of the metals calcium, barium, zinc and aluminium have been advocated as primary stabilizers, optionally supported by costabilizers such as, for example, organic phosphites, imino compounds, epoxy compounds, polyhydric alcohols of 1,3-diketones. Unfortunately, however, these stabilizer systems do not impart adequate initial stability and/or adequate long-term stability to the molding compositions sought to be stabilized. In particular, undesirable premature termination of long-term stability. reflected by the sudden blackening of the polyvinylchloride molding compositions, is observed when stabilizer combinations which contain unduly large quantities of zinc soaps, are used to improve initial stability. This sudden blackening is known as zinc burning. Accordingly, there is a need for compounds which will provide an initial and/or long-term effect to stabilizer systems for molding compositions based on polyvinylchloride may be distinctly improved.